The present invention relates to electrochemical batteries, and, more specifically, to lithium-metal batteries. More particularly, the present invention relates to methods and compositions that enhance the cycle life and shelf life of lithium-metal batteries, and, especially, lithium-active sulfur batteries. The present invention has applications in the fields of electrochemistry and battery technology.
Lithium battery technology continues to be an attractive option for providing light-weight, yet powerful energy sources. Lithium-sulfur secondary batteries are especially well suited to continuing market demands for more powerful and highly portable electronic devices. Examples of such batteries include those disclosed by De Jonghe, et al., in U.S. Pat. Nos. 4,833,048 and 4,917,974; and by Visco, et al., in U.S. Pat. No. 5,162,175. Nevertheless, the batteries described in these, and other references, have serious limitations (Rauh 1979; De Gott 1986). In particular, batteries using sulfur or polysulfide electrodes in combination with lithium, such as the Li2Sx, batteries described by Peled and Yamin in U.S. Pat. No. 4,410,609, have suffered from poor cycling efficiencies (Rauh 1989).
Many of these difficulties are addressed by the batteries described in U.S. Pat. Nos. 5,523,179 and 5,532,077, both to Chu, each of which is incorporated herein by reference in its entirety and for all purposes. Briefly, the ""179 and ""077 patents describe solid-state batteries that comprise a lithium electrode in combination with an active sulfur-containing electrode. An xe2x80x9cactive sulfurxe2x80x9d electrode is an electrode comprising elemental sulfur, or sulfur in an oxidation state such that the sulfur would be in its elemental state if the electrode was fully charged. The technology described in these patents is an important advance in lithium battery technology, in particular by describing batteries having large energy densities and good cycling performance.
The cycle life and shelf life of lithium-sulfur batteries is limited by the slow degradation of the lithium electrode surface arising from the formation of dendritic and/or high surface area xe2x80x9cmossy lithiumxe2x80x9d. To compensate for active lithium loss, extra lithium must be provided for the lithium electrode increasing the cost and weight of the battery. The use of additional metals also increases the burden of disposing the battery as additional toxic materials must be processed. Mossy lithium can also present a fire hazard by creating fine particles of lithium metal that can ignite on contact with air.
Various attempts have been made to provide lithium batteries having long cycle life and improved stability of the lithium metal anode. To minimize the growth of lithium dendrites, stabilize a lithium metal anode, and improve lithium cycling efficiency, one approach has been to add a metal to the lithium to form a solid metal-lithium alloy electrode. For example, aluminum may be added to the lithium to form a solid aluminum-lithium alloy electrode (Rao 1977). However, as described in Huggins, et al., U.S. Pat. No. 4,436,796, solid lithium-metal alloys such as Lixe2x80x94Al or Lixe2x80x94Si exhibit lower surface kinetics and lose there charge capacities after prolonged cycling. In particular, some types of solid Lixe2x80x94Al alloy electrodes, suffer from problems of shape and mechanical instabilities as well as manufacturing difficulties. Further, as described in Kawakami, et al., U.S. Pat. No. 5,698,339, for use in a rechargeable lithium battery, use of a lithium alloy such as lithium-aluminum alloy as an anode is not practical because the lithium alloy is difficult to fabricate into a spiral form. Therefore, it is difficult to produce a spiral-wound cylindrical rechargeable battery. Further, desirable charging and discharging cycle life or energy density for a rechargeable battery is not easily obtained using lithium-alloys as the anode.
Thus, to. take advantage of the stabilizing properties of lithium-metal alloys, which may improve battery cycle life and shelf-life, there remains a need to improve the utilization of lithium-metal alloys in the design of lithium electrodes. The present invention meets these and other needs.
The present invention provides anode material stabilized with a metal-lithium alloy including aluminum-lithium alloy and battery cells comprising such anodes. In one embodiment, the present invention includes an electrochemical cell having a negative electrode (anode) and a sulfur electrode including at least one of elemental sulfur, lithium sulfide, and a lithium polysulfide. The anode includes a lithium core and an aluminum-lithium alloy layer over the lithium core. In another embodiment, a surface coating, which is effective to increase lithium cycling efficiency and anode stability towards electrolyte components during cell storage, is formed on the electrode. In a more particular embodiment, the lithium electrode is in an electrolyte solution, and, more particularly, an electrolyte solution including either elemental sulfur, a sulfide, or a polysulfide where the surface coating is comprised of Al2S3.
One aspect of the present invention provides an electrochemical cell that generally can be characterized as including: 1) a lithium anode and 2) a sulfur electrode including at least one of elemental sulfur, lithium sulfide, and a lithium polysulfide, where the anode has an aluminum-lithium metal alloy layer including a surface coating that is effective to increase lithium cycling efficiency and anode stability towards components of electrolyte during storage of said electrochemical cell. Typically, the anode may be in an electrolyte solution where the electrolyte solution contains elemental sulfur, a sulfide or a polysulfide. In a specific embodiment, the electrolyte solution may contain dioxolane. In some embodiments, the surface coating comprises Al2S3, a film based on poly(dioxolane), products of electroreduction of electrolyte components or a reaction product of aluminum-lithium metal alloy and polysulfides or elemental sulfur.
Another aspect of the present invention provides a lithium anode for use in an electrochemical cell that may be generally characterized as including: 1) a lithium metal layer and 2) a metal-lithium alloy layer substantially thinner than the lithium metal layer where the metal-lithium alloy layer is effective to increase the lithium cycling efficiency and anode stability during prolonged storage of the electrochemical cell. Additionally, the anode may include a surface coating on the metal-lithium alloy layer where the surface coating is a reaction product of aluminum, elemental sulfur and polysulfides, a reaction product of aluminum-lithium alloy, elemental sulfur and polysulfides, or an Al2S3. In a particular embodiment, the metal-lithium alloy layer is between 0.05 and 10 microns thick. In specific embodiments, the metal in the metal-lithium alloy layer is selected from the group consisting of Al, Mg, Bi, Sn, Pb, Cd, Si, In, Ag, and Ga and the anode may be in an electrolytic solution containing elemental sulfur or polysulfides.
Another aspect of the invention provides a method of forming a lithium anode with a lithium metal alloy layer including a surface coating. The method may be characterized as including: 1) depositing a metal layer on an outer surface of the lithium foil sheet, 2) alloying the lithium electrode and the metal layer on the outer surface of the lithium foil to form a metal-lithium alloy layer and 3) forming a surface coating on the metal-lithium alloy layer, wherein the anode is effective to increase the cycling efficiency of lithium and anode stability towards components of electrolyte during storage the electrochemical cell. In a specific embodiment, the metal layer is aluminum and the metal-lithium alloy layer is an aluminum-lithium alloy layer. In other embodiments, the metal layer is selected from the group consisting of Mg, Bi, Sn, Pb, Cd, Si, In, Ag, and Ga and the metal-lithium alloy layer is a Mgxe2x80x94Li alloy layer, a Bixe2x80x94Li alloy layer, a Snxe2x80x94Li alloy layer, a Pbxe2x80x94Li alloy layer, a Cdxe2x80x94Li alloy layer, a Sixe2x80x94Li alloy layer, an Inxe2x80x94Li alloy layer, an Agxe2x80x94Li alloy layer or a Gaxe2x80x94Li alloy layer.
In other embodiments, the metal layer may be deposited using sputtering, plasma deposition, rolling or physical deposition where the metal layer is between about 0.05 to 10 microns thick. The lithium electrode may be in an electrolyte solution where the electrolyte solution contains at least one of elemental sulfur, a sulfide, and a polysulfide. The surface coating of the anode may be formed by the reaction of elemental sulfur, the sulfide or the polysulfide with a metal-lithium alloy where the surface coating of the anode may be comprised of a reaction product of elemental sulfur, the sulfide or the polysulfide with the metal-lithium alloy. Additionally, the surface coating of the anode may be formed by the reaction of elemental sulfur, the sulfide or the polysulfide with the metal layer where the surface coating of the lithium electrode is a reaction product of elemental sulfur, the sulfide or the polysulfide with the metal-lithium alloy. Further, the surface coating of the may be formed by the reaction of an chemical species or a compound in said electrolyte with the metal-lithium alloy layer where the surface coating of said anode is comprised of a reaction product of the chemical species or the compound in the electrolyte with the metal-lithium alloy layer.
In another embodiment, the method may include passing a current through the lithium electrode to enable the reaction of the chemical species or the compound with the metal-lithium alloy layer. The surface coating of the anode may be formed by the reaction of an chemical species or a compound in the electrolyte with the metal layer on the anode where the surface coating of the anode is comprised of a reaction product of the chemical species or the compound in said electrolyte with said metal layer. Further, the method may include: 1) prior to depositing a metal layer on the lithium foil, depositing a first metal layer on a inert substrate for example an aluminum film on a plastic sheet, 2) alloying the metal layer on the substrate with lithium foil to form a metal-lithium alloy layer on the inert substrate, 3) removing the metal-lithium alloy layer from the inert substrate. Another aspect of the invention provides a method of forming a lithium anode with an aluminum-lithium alloy layer for an electrochemical cell. The method may be characterized as including: 1) pressing at a first pressure an aluminum foil sheet to an outer surface of lithium foil sheet to form the aluminum-lithium alloy layer of the anode surface, 2) while pressed at the first pressure, heating the anode at a first temperature for a first time in a first gas, 3) pressing at a second pressure the anode in a second gas for a second time and 4) heating the anode at a second temperature for a third time. In a specific embodiment, prior to pressing the aluminum foil to the outer surface of the lithium foil sheet, the method may include polishing the outer surface of the lithium foil sheet. In another embodiment, the method may include prior to the formation of the aluminum-lithium alloy layer, forming a surface coating on the aluminum-lithium alloy layer. In yet another embodiment, the method may include passing a current through the lithium electrode to enable the formation of the surface coating.
Another aspect of the present invention provides a battery cell which may be generally characterized as including: 1) a positive electrode comprising a mixture of an electrochemically active material, and an electronically conductive material, 2) an anode comprising a lithium core with a metal-lithium alloy layer including a surface coating that is effective to increase the lithium cycling efficiency and anode stability towards components of electrolyte during storage of said electrochemical cell, and 3) an electrolyte electronically separating the positive and negative electrodes.
These and other aspects and advantages will become apparent when the Description below is read in conjunction with the accompanying Drawings.